As is known, platinum, together with silver and gold, is one of the preferred metals widely used for the production of jewellery articles, among other things owing to its chemical stability and its aesthetic properties and colour which remain unchanged over time. Platinum, moreover, is distinguished by a high degree of malleability and ductility. However, generally it cannot be used in the pure state, but as an alloy together with other metals, such as palladium and indium, which increase its mechanical strength and allow it to be worked more easily.
Nowadays platinum and its alloys are widely used for setting precious stones, in particular diamonds, mainly because of their somewhat “neutral” colour which increases the brilliance of the stones. Consequently, as is known, nowadays the most famous diamonds are nearly all set in platinum mountings.
In the jewellery sector the platinum content of a product is usually indicated by means of an index—referred to as “purity index” —which is generally expressed as a fraction, i.e. thousandth part, of a metal with respect to the total weight of the said product. The platinum alloys which are currently used in the precious stones industry have standardized purity indices, i.e. 850/1000, 900/1000 or 950/1000. The same index may also be expressed in carats (K), in which case the fraction of metal with respect to the overall weight of the product is expressed as a twenty-fourth part ( 1/24), instead of a thousandth part, as indicated by the purity index.
For each platinum alloy, the choice of purity and metals which make up the alloy together with platinum, commonly referred to in the technical sector as “alloying elements”, generally depends on commercial factors and the type of product which is to be obtained.
The alloying elements which are currently most frequently used are cobalt (Co), iridium (Ir), gold (Au), palladium (Pd), rhodium (Rh) and rhutenium (Ru). During the last few years, some of these metals, in particular palladium, rhutenium and rhodium, have reached prices which are comparable to, if not higher than, than that of platinum, so much so that they have resulted in a considerable increase in the final prices of the jewels made with these alloys.
A major drawback associated with the use of these alloys consists in the high melting temperature of platinum (1769° C.) which, as is known, complicates any process associated with the metallurgy of platinum and its alloys.
Moreover, the metallurgical plants and apparatus which are commercially available in the sector in question are almost exclusively intended for the goldsmith's sector, more specifically for the processing of gold, silver and alloys thereof which are distinguished by very simple metallurgical properties compared to those of platinum and its alloys.
Consequently, the plants envisaged in the gold and silver processing sector have proved to be totally inadequate for performing metallurgical operations on an industrial scale at the high temperatures which are required for platinum alloys. For example, the furnaces which are traditionally used for the processing of gold and silver alloys usually operate at temperatures which are much lower than those envisaged for platinum or its alloys.
The welding of semifinished articles made of platinum alloys is equally problematic owing to the high melting temperature of platinum which is not normally reached by the apparatus intended for the welding of gold alloys.
Welding with weld material at a lower melting temperature, such as for example braze-welding, although it is able to solve the problem associated with the high welding temperatures of platinum, is not convenient from a cost point of view considering the processing steps as a whole. In fact the use of impure weld material results in variations in the purity of the finished product, which must be necessarily taken into account, in some cases using complex calculations, in order to determine the final purity. For example, in order to compensate for the loss of purity due to the use of weld material, it may be envisaged initially using platinum alloys with fairly high fractions of platinum.
Advantageously, for practical reasons, the purity indices of the semifinished articles would need to be determined beforehand, at the time of melting, by calculating the number of welding operations necessary for achieving the end product. However, the management of a high and variable number of melting operations involving different purity indices is generally not practical and cost-effective.
It is known that the platinum alloys currently available on the market are distinguished, in mechanical terms, by a low elongation capacity, which limits the production of elongate semifinished articles in particular for chains, bands and the like. For this same reason, the manufacture of so-called “hollow” products, such as hollow chains, namely products which are able to house internally cores made of other materials, is also very problematic.
On average, platinum alloys of the known type have cold elongation values equal to 10% and therefore significantly lower than the elongation values of gold alloys which, depending on the purity and the alloying elements, are generally distinguished by elongation values ranging between 20% and 60%.
At present the elongation drawbacks of platinum alloys are partly overcome by using a fairly laborious alloy processing method which envisages alternating cold-rolling steps with annealing steps. In this way, during each annealing step, the internal tensions and the distortions in the crystal structure of the alloy produced during the prior rolling step are reduced, thus restoring the initial mechanical elongation properties prior to each new rolling step.
The process for the production of platinum semifinished articles is however, for this reason, penalized considerably in terms of time and cost (and therefore ultimately in terms of productive efficiency), compared to the corresponding production process for gold or silver semifinished articles.
As is known, major problems are also encountered with regard to recycling of the waste resulting from the processing of platinum alloys. In fact, pure and simple remelting of the waste does not eliminate the substances catalyzed by platinum during the course of the processing which it undergoes. Consequently, the waste is practically unusable and this obviously constitutes a major drawback from the point of view of cost-efficiency.
The well-known catalytic properties of platinum, which, as already mentioned, result in increased impurities in its processing waste, also give rise to difficulties which are encountered in the processes for diamond-cutting semifinished articles made of platinum or alloys thereof. The diamond-cutting process consists in cutting and mechanically removing, in the cold state, surface layers from products using diamond-tipped tools. By means of this process, which is practically indispensable in the precious stones industry, it is possible to obtain particularly shiny surfaces and light effects which enhance the semifinished article.
The traditional tools used in the diamond-cutting of precious metals such as gold and silver have proved to be totally inadequate for performing the diamond-cutting of platinum since their diamond-coated parts are subject to an exceptional degree of wear, which is not even comparable to that encountered with gold. This wear is generated not so much by the mechanical abrasive action produced by the platinum surface, which is very soft, but by the chemical attack produced by the platinum catalysis processes on the carbon crystal structure of the diamond.
Owing to the rapid wear of the diamond parts in fact, the diamond-cutting of products made of platinum or its alloys is currently unsustainable from a cost point of view.